240 Fish as feed inputs for aquaculture – Practices, sustainability and implications
Berggren and Ganning, 2003). In some cetaceans, the proportion of feedfish reported
in the diet is minimal, but in Scottish waters, sand eels constitute 58 percent by weight
of the stomach content in harbour porpoises and 49 percent by weight of the stomach
content in the common dolphin. Other feed-fish species, sprat and Norway pout, were
less than 1 percent by weight (Santos et al., 1995). In Kattegat and Skagerrak Seas,
feedfish (mainly sprat and herring) constitute 13 percent by weight of the contents in
juveniles’ stomachs and 10 percent by weight in adults’ stomachs (Borjesson, Berggren
and Ganning, 2003). Sand eels contribute 86.7 percent to the diet by weight of Minke
whale in the North Sea and further north, into the Norwegian Sea, the diet of Minke
whales is dominated by spring-spawning herring (Olsen and Holst, 2001). The
differences in the diet composition reflect the local foraging of cetaceans. Industrial
fisheries in the North Sea may, therefore, impact marine mammal populations by
altering their food supply in certain areas. It is, therefore, important to consider the
local availability of feedfish to cetaceans and their ability to switch to other prey if
the stocks are depressed, when assessing the effects of feed-fish fisheries on marine
mammals. This, however, has yet to be demonstrated in any cetacean population.
There is some evidence that there is a link with fisheries and grey seal population
dynamics. The Effects of Large-scale Industrial Fisheries On Non-Target Species
(ELIFONTS) study investigated the grey seal population on the Isle of May, in the
North Sea. Grey seals (Halichoerus grypus) consumed mainly sand eels (Ammodytes
marinus), but the greater sand eel (Hyperoplus lanceolatus) was also taken. For this
study, the proportion of not breeding, but reproductively capable females and the
number of breeding failures among marked animals were positively correlated with
sand-eel catch per unit effort (CPUE) in the southern North Sea in the years
1990–1997. Effects were only seen when the reproductive performances of known
seals were examined in relation to fishery data. It is possible that the reproductive
performance of some seals may be more affected by changes in sand-eel availability
than that of other seals, reflecting either a tendency to specialize on sand-eels or
an inadequacy in hunting behaviour. Also, the body condition of female seals was
positively correlated with CPUE for the local stock area. However, the total number
of pups increased steadily during the study periods and thus, although there appears
to be an interaction between sand-eel abundance and seal breeding success, given the
current state of the populations, this interaction does not appear to be a major factor
explaining variations in seal populations (Harwood, 1999).Ecosystem changes
The complexity of marine systems makes it difficult to identify the effects of predator/
prey removal on other communities. Marine communities often exhibit size-structured
food webs, and changes in the abundance and size composition of populations are
likely to lead to changes in the quantity and type of prey consumed (Frid et al., 1999).
However, these changes may not be predicted by simplistic models of predator-prey
interactions, as they do not take into account prey switching, ontogenetic shifts in diet,
cannibalism or the diversity of species in marine ecosystems (Jennings and Kaiser, 1998;
Jennings, Kaiser and Reynolds, 2001).
Ecological dependence takes account of the ecological linkages in the marine systems.
Ecological dependence is already considered in management advice for sand-eel in the
Shetland area, and sand eel in Sub-area IV, e.g. the kittiwake/sand eel interaction. ICES
(2002) identified several feed-fish stocks for which ecological dependence may need to
be considered further in management advice: sand eel in Division IIIa; Norway pout in
Sub-area IV and Division IIIa; sand eel in Sub-area IV; Norway pout in Division VIIa
and sand eel in Division VIa. However, assessing ecological dependence is problematic,
as evidence for the effects of strong ecological interactions on some stocks, e.g. the
proposed kittiwake/sand-eel interaction, should not be taken as evidence that they are